@misc{cryptoeprint:2010:143,
author = {Gregor Leander},
title = {Small Scale Variants Of The Block Cipher PRESENT},
howpublished = {Cryptology ePrint Archive, Report 2010/143},
year = {2010},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.